**Core Concept**
The process of protein synthesis involves the translation of messenger RNA (mRNA) into a specific sequence of amino acids that form a polypeptide chain. This process occurs on structures called ribosomes, which are composed of two subunits and are found in the cytoplasm of eukaryotic cells.

**Why the Correct Answer is Right**
The correct answer is the cellular component responsible for translating mRNA into a protein sequence. Ribosomes are the site of protein synthesis, where transfer RNA (tRNA) molecules bring amino acids to the ribosome, which then link them together in a specific sequence determined by the mRNA sequence. This process is mediated by the small and large subunits of the ribosome, which work together to translate the mRNA sequence into a polypeptide chain. The ribosome's role in protein synthesis is essential for the production of proteins, which perform a wide range of cellular functions.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the cellular component responsible for protein synthesis. The endoplasmic reticulum (ER) is involved in protein synthesis, but it is not the primary site of translation. The ER is responsible for folding and modifying proteins after they have been synthesized on ribosomes.
* **Option B:** This option is incorrect because it does not accurately describe the cellular component responsible for protein synthesis. The mitochondria are involved in energy production and do not play a direct role in protein synthesis.
* **Option D:** This option is incorrect because it does not accurately describe the cellular component responsible for protein synthesis. The nucleus is involved in DNA replication and transcription, but it is not the site of protein synthesis.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that ribosomes are the site of protein synthesis, and their function is essential for the production of proteins. Disruptions to ribosome function can lead to a range of cellular and systemic problems, including muscle weakness, fatigue, and impaired wound healing.

**Correct Answer:** C.